Separating and recovering the target hydrocarbon gas (e.g., ethylene and 1,3-butadiene) from a mixed gas containing hydrocarbons are known technology.
An example of a hydrocarbon gas to be separated and recovered is ethylene. Ethylene is an important chemical compound as a raw material for various products in synthetic chemical industries, such as ethylene oxide, vinyl chloride, acetaldehyde, styrene, and polyethylene.
Ethylene is generally produced by naphtha cracking or dehydrogenation of ethane. Ethylene is recovered as a distillate fraction having 2 carbon atoms, which contains the target ethylene as well as other compounds, such as ethane. Therefore, it is necessary to selectively separate and recover ethylene from the mixture produced. One method for separating is distillation. However, since the boiling point of ethylene is close to that of ethane, cryogenic separation under high pressure at low temperature is necessary, and as a result, consumes a lot of energy.
A method for separating and recovering ethylene more energy-effectively includes separation by adsorption. When a mixed gas is separated by pressure swing adsorption or temperature swing adsorption, in general, a molecular sieving carbon, zeolite, and the like is used as an adsorption material, and separation is achieved by the differences in its equilibrium adsorption amount or adsorption rate. However, when a mixed gas is separated by utilizing the differences in equilibrium adsorption amount of each component gas, since the conventional adsorption materials cannot selectively adsorb only the gas to be removed, the separation factor becomes smaller, which results in an increase in the size of an apparatus.
Another example of a hydrocarbon gas to be separated and recovered is 1,3-butadiene. 1,3-butadiene is useful compound as, for example, a raw material for the production of a synthetic rubber, as well as an intermediate of enormous compounds. 1,3-Butadiene is generally produced by naphtha cracking or dehydrogenation of butene. In these production methods, 1,3-butadiene is obtained as one component of a mixed gas. Therefore, it is necessary to selectively separate and recover 1,3-butadiene from the products obtained as mixtures. The principal components having 4 carbon atoms in the products may be 1,3-butadiene, isobutene, 1-butene, 2-butene, normal butane, and isobutane. These have the same carbon number and close boiling points, and thus it is difficult to carry out separation by distillation that is usually used in industrial scale.
One of the other separation methods includes extractive distillation. Since this method is an absorption method using polar solvents, so much energy is used when 1,3-butadiene is recovered from the polar solvents. Therefore, separation by an adsorption method is desirable as a method for separating and recovering 1,3-butadiene with reduced energy.
However, since conventional porous materials (Patent Literature 1) have low separation performance, multistage separation is required, which results in an increase in the size of a separation apparatus.
As an adsorption material providing excellent separation performance, a porous metal complex, in which dynamic structural changes occur by an external stimulus, has been developed. When this porous material is used as a gas adsorption material, it has been observed that gas is not adsorbed below a certain pressure, but gas adsorption occurs above a certain pressure, which is a particular characteristic of the material. In addition, it has been observed that there is a characteristic in which a pressure at which adsorption occurs differs depending on the type of gas subjected to adsorption.
When this porous material is applied to, for example, an adsorption material in a pressure swing adsorption-type gas separation apparatus, highly efficient gas separation can be achieved. In addition, the swing width of pressure can be narrowed, which contributes also to energy saving. Furthermore, since it can contribute to the downsizing of a gas separation apparatus, cost competitiveness can be enhanced when a highly pure gas is marketed as a product, and even when a highly pure gas is used within one's own factories, costs necessary for the facility that requires a highly pure gas can be reduced, and as a result, the cost for the production of final products can be reduced.
Patent Literatures 2 to 8 and Non Patent Literatures 1 to 3 disclose a metal complex [Cu2(pydc)2(pyz)] of a copper ion, 2,3-pyrazinedicarboxylate dianion, and pyrazine. Although these documents have reported the adsorption properties of acetylene and methane, they have not mentioned the adsorption and separation of ethylene and hydrocarbon gases having 4 carbon atoms.